The economic advantages of transmitting information in the form of optical signals have been realized in commercial systems. In currently used optical transmission systems, the optical signals are converted to electronic ones before processing occurs. Such processing involves the use of standard electronic devices.
Designs for future optical communication systems go beyond the simple transmission of information on an optical carrier, and include the processing of signals while still in optical form. In the next generation of optical communication systems, it is envisioned that optical signals will be processed without conversion to electronic signals. Such optical processing will require optical devices which are analogous to those devices, such as amplifiers, modulators, filters, multiplexers, demultiplexers, for example, which are used for processing electronic signals.
An easily manufacturable optical filter having a bandwidth between about 100 MHz and a few tens of gigahertz with low insertion loss would be an important component in wavelength multiplexing as well as in many other applications. It appears that the most promising approach to such a device is a fiber Fabry-Perot interferometer which may be referred to as an FFP.
A Fabry-Perot interferometer is an optical device which can be used to process optical signals and includes two mirrors with a cavity therebetween. The Fabry-Perot interferometer is discussed in most of the classic texts and its operation is well understood. See, for example, Born & Wolf, Principles of Optics, MacMillan, 1959, pages 322-332. An exemplary Fabry-Perot structure comprises a region bounded by two plane, parallel mirrors. The structure exhibits low loss, that is, it passes only particular wavelengths, for which the cavity is said to be in resonance-a condition obtained by adjusting appropriately the cavity parameters. At resonance, the cavity passes a series of approximately equally spaced wavelengths. The spacing between these wavelengths, called the free spectral range (FSR) or tuning range of the cavity, is a function of the spacing between the mirrors and the index of refraction of the medium between the mirrors. The tuning range of a Fabry-Perot interferometer is equal to c/2nl.sub.c where l.sub.c is used to designate the length of the cavity. Accordingly, the shorter the cavity, the larger the tuning range. The bandwidth is largely determined by the reflectivity of the mirrors; however, other sources of loss and reflections can affect bandwidth. Another parameter which is designated finesse (F) is equal to the quotient of the tuning range divided by the bandwidth.
The use of Fabry-Perot cavities as filters, for example, to process optical signals is well known. However, the application of such devices to the processing of optical signals in commercial optical fiber communication systems has been hampered by, among other constraints, the lack of practical designs which have suitable characteristics, such as low loss when used with optical fibers and appropriate values of free spectral range. Nevertheless designs that more closely meet the needs of a commercial fiber system have been suggested. For example, in Electronics Letters, Vol. 21, No.11, pp. 504-505 (May 23, 1985), J. Stone discussed a fiber Fabry-Perot interferometer design in which the cavity was an optical fiber waveguide with mirrored ends. The free spectral range of the resulting cavity is determined by the length of the fiber segment. Accordingly different free spectral ranges can be obtained by using fiber segments of different lengths. The cavity can be tuned over one free spectral range by changing the cavity optical length by one-half the wavelength value of the light entering the cavity. In this way, the cavity can be tuned to resonate at, and therefore transmit light of different wavelength values. To obtain such tuning, the cavity length can be changed, for example, by means of a piezoelectric element attached to the fiber, which, when activated, will stretch the fiber and increase the associated cavity optical length accordingly. Fiber Fabry-Perot interferometers can be made with a finesse up to a value of 500 with relatively low insertion loss, using separately attached mirrors.
In an article entitled "Pigtailed High-Finesse Tunable Fiber Fabry-Perot Interferometers With Large, Medium and Small Free Spectral Ranges", authored by J. Stone and L. W. Stulz, appearing in the Jul. 16, 1987 issue of Electronics Letters beginning at page 781, the authors demonstrated that fiber Fabry-Perot interferometer devices with any required bandwidths can be fabricated from one of three types of structures reported in that article. Tuning is accomplished by stretching the fiber.
A so-called Type 1 structure reported in the above-identified article by Stone and Stulz is a fiber resonator. Mirrors are deposited on both ends of a continuous fiber and tuning is achieved by changing the optical length of the fiber. This type of fiber Fabry-Perot interferometer generally is limited to a length greater than 1 to 2 cm which equates to a free spectral range on the order of 10 to 5 GH.sub.z. Although no alignment is required inside the cavity, the bandwidth range is limited to less than 100 MHz for a finesse of 100 and an l.sub.c of 1 cm.
Among the advantages of the Type 1 Fabry-Perot interferometer is the fact that the cavity comprises an optical fiber which is a waveguide. This eliminates deleterious diffraction effects present in long Fabry-Perot cavities which are not waveguides. The elimination of the deleterious diffraction effects is associated with the guiding characteristics of the fiber. However, the difficulty of working with and stretching small lengths of optical fiber precludes large values of free spectral range when using a Type 1 Fabry-Perot. As a result, the usefulness of the Type 1 Fabry-Perot design is somewhat limited.
A Type 2 fiber Fabry-Perot inteferometer is a gap resonator with mirrors deposited on adjacent end faces of two optical fibers. In this type of filter, the defraction loss between the fibers limits the resonator gap to less than 10 .mu.m which corresponds to a free spectral range greater than 10,000 GH.sub.z.
Large free spectral ranges can be obtained by using a Type 2 Fabry-Perot interferometer in which the cavity comprises a small gap. However, because of diffraction losses, wider gap cavities are less practical. and therefore the Type 2 Fabry-Perot interferometer is not adequate for applications which require the smaller free spectral ranges otherwise associated with larger gaps. Unacceptable losses result from gaps in excess of 10 .mu.m.
A Type 3 structure reported on by Stone and Stulz is an internal waveguide resonator. A mirror film is applied to an end of one external fiber disposed in the passageway of a glass or ceramic ferrule and another to one end of an internal waveguide. The ferrule which supports the external fiber is movably mounted in a sleeve in which also is disposed the internal waveguide and another ferrule in which an optical fiber is disposed. A relatively small gap separates the mirrored end of the external waveguide and an unmirrored end of the internal waveguide. Scanning is accomplished by changing the spacing of the small gap between the mirror film at the end of the external fiber and the internal waveguide. The free spectral range is determined by the length of the internal waveguide which can be as short as 1 mm or less. An anti-reflection coating may be applied to the non-mirrored end of the internal waveguide. Although the Type 3 fiber Fabry-Perot interferometer covers the most practical range of frequencies, it may be somewhat difficult to manufacture because of the lengths of the internal waveguide.
In each of the above-described three types of Fabry-Perot interferometers, the fiber ends are disposed in glass or ceramic ST.RTM. connector ferrules. Afterwards, the ends are polished and coated with multi-layer dielectric mirrors. The ferrules are held in alignment with either a split zirconia sleeve or a solid zirconia sleeve and the assembly is mounted in a piezoelectric shell which is attached such as, for example, with an epoxy material to the ferrules. Should a fiber connection be needed, it may be carried out by connecting ST or rotary splice connectors to the outer ferrule ends for the Type 1 or to fiber pigtails for Types 2 or 3.
Other techniques are known to minimize diffraction losses in large gap cavities, such as the use of expanded beams or concave mirrors which keep the beam confined by refocusing. However, those techniques involve arrangements which are difficult to implement with optical fibers.
The problem is to obtain a very sharp narrow band optical filter with cavity lengths that span from a few microns to several millimeters which correspond to bandwidths between a few tens of gigahertz and approximately 100 MH.sub.z with a stable repeatable design that is relatively easy to manufacture with high yield. Also the sought after device is an optical filter which is tunable and has low loss. Still further, the sought after optical filter has a relatively high extinction or contrast ratio, that is, one which has a large difference between the passband and the stopband insertion loss.